Seawater corrosion mechanisms

Water injection from seawater or fresh water sources contributes to the souring of oil fields with H2S usually resulting in an increase in the corrosion rate, which sometimes requires a complete change in corrosion strategy. These water sources may necessitate biocide injection and will require deaeration to avoid introducing a new corrosion mechanism into the existing system. Tertiary recovery techniques are often based on miscible and immiscible gas floods. These gas floods invariably contain a... 

Tinbronze CuSnlO From C90200 Seawater-, corrosion- and cavitation-resistant, tough alloy with good wear resistance. Well suited for pumps and turbine components, fittings for high mechanical loads, and thin-walled castings. 

Li Y, Corrosion Mechanism and Reinforced Protection of Hot-Dipped Coated Steel Wire in Seawater, Ph.D. 

Pseudomonas spp. are IRB species reported to have corrosive effects. " - However, there is an increasing body of evidence that IRB could actually slow down corrosion. Hernandez et al. reported that in the presence of bacteria like aerobic Pseudomonas sp. and facultative anaerobic Serratia marcescens in synthetic seawater, corrosion of mild steel is inhibited. The effect seemed to disappear with time in natural seawater. Although no particular mechanism was proposed to address this phenomenon, it seems that it is related to the biofilm produced, thus reducing the contribution of factors such as diffusion gradients that normally enhance corrosion. 

S] Syrett, B. C., The Mechanism of Accelerated Corrosion of Copper-Nickel Alloys in Sulfide-Polluted Seawater, Corrosion Science. Vol. 21, 1981, p. 187. 

The most widely used cabinet test is the neutral salt spray (Fog) test (ASTM B 117), which consists of a fog of 5 % sodium chloride within the chamber at 35 C [46. Controversy exists over the validity of B 117 as a performance test because corrosion mechanisms are not always the same as those observed in automobile service. Also, not all materials can be successfully evaluated in the test. However, the value of the salt spray test as a quality assurance test is well documented [46]. Several modifications to the salt spray test have been developed including acetic acid salt spray (ASTM G 85, Annex 1), copper accelerated acetic acid salt spray (ASTM B 368), acidified synthetic seawater fog (ASTM G 43, Method of Acidified Synthetic Seawater (Fog) Testing), and modified salt spray (ASTM G 85). ASTM G 85 also includes cyclic tests. 

EIS measurements clarifying that the corrosion process is mainly charge-transfer controlled and no change in the corrosion mechanism occurred due to the inhibitor addition to seawater. It also indicates that the Ra values increase with addition of inhibitor whilst, the capacitance values decrease indicating the formation of a surface film. The EIS measurement also confirms the similar corrosion process and mechanism occurs in PP measurements. According to LPR data, the values of Rp of Al-Mg-Si after addition of the studied inhibitors increase with the following order NH < VL < TS. 

In general, cathodic protection can be applied to any material susceptible to corrode, but this method is commonly used to protect carbon steel stmctures in diluted or alkaline electrolytes, such as seawater and soil The corrosion mechanism of iron or carbon steel was introduced in Section 13, Chapter one and Chapter four. Nevertheless, the corrosion product may be an unstable ferrous hydroxide [Fe OH) solid compound, which reacts in the environment to form ferric hydroxide compound [Fe (OH) ] or hydrated ferric oxide (FezOs-SH ) known as mst. The formation of this corrosion product is avoidable using cathodic protection. However, careful application of an external potential to a structure must be considered because hydrogen evolution may be induced leading to destmction of any coating and Hydrogen Embrittlement [1]. 

Corrosion behavior. General corrosion rates for 90-10 and 70-30 copper-nickel alloys in seawater are low, ranging between 25 and 2.5 p,m y 1. For the majority of applications, these rates would allow the alloys to last the required hfetime, and there would be little probability of their premature failure in service due to such a corrosion mechanism. ... 

The stress-corrosion cracking (SCC) properties of graphite/aluminum MMCs are discussed in Ref 19 and 20. Based on evaluation of a limited nutrber of specimens, an initial stress-dependent corrosion mechanism was reported for graphite/aluminum, which then shifted to a corrosion-dominated failure as the exposure in seawater increased (>100 h) (Ref 19). In another study, a corrosion-dominated mechanism was also noted at longer exposure times, but it was suggested that the failures were creep related as well (Ref 18). 

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

In the tidal zone and the spray zone (known as the splash zone), cathodic protection is generally not very effective. Here thick coatings or sheathing with corrosion-resistance materials (e.g., based on NiCu) are necessary to prevent corrosion attack [4]. The coatings are severely mechanically stressed and must be so formed that repair is possible even under spray conditions. Their stability against cathodic polarization (see Section 17.2), marine growths, UV rays and seawater must be ensured [4,5]. 

Pitting corrosion always remains a worthy subject for study, particularly with reference to mechanism, and the problem conveniently divides into aspects of initiation and growth. For 6061 alloy in synthetic seawater, given sufficient time, pit initiation and growth will occur at potentials at or slightly above the repassivition potential . In an electrochemical study, it was found that chloride ions attack the passive layer as a chemical reaction partner so that the initiation process becomes one of cooperative chemical and electrochemical effects . 

The application of fracture mechanisms has shown the acceleration of the crack under fretting conditions in the first 100 fim of its growth . Corrosive conditions, e.g. seawater, further accelerate the initial crack growth rate. ... 

This type of damage is dealt with comprehensively in Section 8.8. It can be particularly severe in seawater giving rise to cavitation corrosion or cavitation erosion mechanisms, and hence can be a considerable problem in marine and offshore engineering. Components that may suffer in this way include the suction faces of propellers, the suction areas of pump impellers and casings, diffusers, shaft brackets, rudders and diesel-engine cylinder liners. There is also evidence that cavitation conditions can develop in seawater, drilling mud and produced oil/gas waterlines with turbulent high rates of flow. 

Evans, T. E., Mechanisms of Cathodic Protection in Seawater . In Cathodic Protection Theory and Practice, 2nd International Conference, Stratford-upon-Avon, June (1989) Choate, D. L., Kochanezyk, R. W. and Lunden, K. C., Developments in Cathodic Protection Design and Maintenance for Marine Struaures and Pipelines , NACE Conference on Engineering Solutions for Corrosion in Oil and Gas Applications, Milan, Italy, November (1989) not included in Proceedings... 

P. Chandrasekaran and S.C. Dexter, Mechanism of Potential Ennoblement on Passive Metals by Seawater Biofdms, Paper 493, Corrosion 93 (Houston, TX), NACE International, 1993... 

Carbon itself has been successfully used as a biomaterial. Carbon based fibers used in composites are known to be inert in aqueous (even seawater) environments, however they do not have a track record in the biomaterials setting. In vitro studies by Kovacs [1993] disclose substantial electrochemical activity of carbon fiber composites in an aqueous environment. If such composites are placed near a metaUic implant, galvanic corrosion is a possibility. Composite materials with a polymer matrix absorb water when placed in a hydrated environment such as the body. Moisture acts as a plasticizer of the matrix and shifts the glass transition temperature towards lower values [Delasi and Whiteside, 1978], hence a reduction in stiffness and an increase in mechanical damping. Water immersion of a graphite epoxy... 

Seawater is generally considered as an aggressive environment, and carbon steel as a mechanically resistant material but not very resistant to corrosion on the other hand, stainless steel is considered as a corrosion-resistant but expensive material. Then, in order to design a seawater reservoir, one might think of a carbon steel reservoir plated with stainless steel as an intermediate solution, of intermediate cost, but able to exploit, on one hand, the corrosion resistance of stainless steel and, on the other, the mechanical strength of carbon steel. Quite likely, according to the most widespread opinion, a nonexpert designer would put the stainless steel in contact with seawater. 

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